Watts vs. Amps

[GMichael]

Watts vs. Amps

I have seen many times that some speakers do not need Watts as much as they need Amps. This confuses me each time I read it. When I studied electricity in high school & college we were told that Watts equal Amps times Volts: (W = A x V), And since the Volts are staying the same shouldn't an increase in Watts cause a proportional increase in Amps and vice versa?
Am I being too simple with my thinking? There must be something I'm missing right?
Please let me know what I am not seeing. Oh, and please use small words and talk/type slowly for me.

Tanx.

[GMichael]

Anyone?

[kexodusc]

Hi GM,
I'll try to keep this simple (mostly because it's the only way I can understand it).

Voltage doesn't really stay constant. The voltage waveform is amplified at the input stage of an amplifier ("Voltage gain") by a fixed multiple...then the amp supplies the appropriate current to drive that voltage into the load (speaker).

Let's say you have an amp with a gain of 10, into a load of 8 ohms (your speaker, which isn't really an 8 ohm resistive device, but work with me here). You get 10 volts at the output for evey 1 volt at the input.

voltage/resistance = amps . Ohm's law states:
10/8 = 1.25 amps.

Power = volts x current so in this case it's 10 x 1.25 = 12.5 watts. (which is actually quite a bit).

It's not really the increase in watts that leads to the increase in amps, but rather an increase in amps causing an increase in watts.

Now the above assumed the speaker was an 8 ohm resistive load...that's not true. Most speakers are anything but fixed resistive devices. They have dips and peaks. An 8 ohm nominal speaker can have a peak above 20 ohms (or higher) and a dip to 4 ohms (or lower) at certain frequencies.

So you're playing a test tone, of constant SPL on your speaker, which is requiring all 12.5 watts you're sending. Suddenly, the test tone lowers in frequency (a bass note), the speaker's impedance drops to 4 ohms. Now, all of a sudden, that 10 volts is being fed into a 4 ohm device, so:
10/4 = 2.5 amps are required.

Cool right?

But what happens if your amp's power supply can only feed your amplifier 2.0 amps?
That's when you take a ride to clip-city central.

Current just isn't there.

Also, the amp's power supply rail voltages put limits on the maximum output voltage.
So, say your input was 2 volts, then voltage gain of 10 would make 20 V the amp's new output voltage. If it couldn't output 20 volts because the rails were limited to 15 V, the amp also clips.
Clipping is bad, mmmkay? You shouldn't drive your amp to clipping, mmmkay...

[GMichael]

Hi GM,
I'll try to keep this simple (mostly because it's the only way I can understand it).

Voltage doesn't really stay constant. The voltage waveform is amplified at the input stage of an amplifier ("Voltage gain") by a fixed multiple...then the amp supplies the appropriate current to drive that voltage into the load (speaker).

Let's say you have an amp with a gain of 10, into a load of 8 ohms (your speaker, which isn't really an 8 ohm resistive device, but work with me here). You get 10 volts at the output for evey 1 volt at the input.

voltage/resistance = amps . Ohm's law states:
10/8 = 1.25 amps.

Power = volts x current so in this case it's 10 x 1.25 = 12.5 watts. (which is actually quite a bit).

It's not really the increase in watts that leads to the increase in amps, but rather an increase in amps causing an increase in watts.

Now the above assumed the speaker was an 8 ohm resistive load...that's not true. Most speakers are anything but fixed resistive devices. They have dips and peaks. An 8 ohm nominal speaker can have a peak above 20 ohms (or higher) and a dip to 4 ohms (or lower) at certain frequencies.

So you're playing a test tone, of constant SPL on your speaker, which is requiring all 12.5 watts you're sending. Suddenly, the test tone lowers in frequency (a bass note), the speaker's impedance drops to 4 ohms. Now, all of a sudden, that 10 volts is being fed into a 4 ohm device, so:
10/4 = 2.5 amps are required.

Cool right?

But what happens if your amp's power supply can only feed your amplifier 2.0 amps?
That's when you take a ride to clip-city central.

Current just isn't there.

Also, the amp's power supply rail voltages put limits on the maximum output voltage.
So, say your input was 2 volts, then voltage gain of 10 would make 20 V the amp's new output voltage. If it couldn't output 20 volts because the rails were limited to 15 V, the amp also clips.
Clipping is bad, mmmkay? You shouldn't drive your amp to clipping, mmmkay...

OK, this all makes sense to me.
They only thing that still gets me is this. If an amplifier is rated (lets say rated means that it is) at say 25 watts, shouldn't it be able to procuce the amps required to get to that power? So when you get to the dip in resistance to 4 ohms, (10/4 = 2.5 amps are required x 10v = 25 watts) Wouldn't an amplifier that can (really can) yield 25 watts also be able to produce 2.5 amps (with these assumed numbers)? Or is it that the ratings are different for watts as they are for amps? Maybe an amplifier that can put out 25 watts rms means that 25 is just the average with no head room? And that the amps are cold and hard (need to be there or else) rated?
Am I at least heading in the right direction?

[kexodusc]

.
They only thing that still gets me is this. If an amplifier is rated (lets say rated means that it is) at say 25 watts, shouldn't it be able to procuce the amps required to get to that power?

Short answer yes....long answer...depends on what "rating scheme" is used. Most ratings for Home theater are using the qualifier 8 ohm loads. Some amplifiers (especially receivers) use a less strict rating standard. Watts alone isn't a great measurement unless you qualify it with frequency range, resistive load, and distortion specs.
But yeah, all things equal, a 25 real watt amp should have enough amps to do this. The problems come when you start adding low impedance speakers. Even some 8 ohm speakers might have very low impedance dips. This can suck out current pretty fast and put some strain on the amplifier.

So when you get to the dip in resistance to 4 ohms, (10/4 = 2.5 amps are required x 10v = 25 watts) Wouldn't an amplifier that can (really can) yield 25 watts also be able to produce 2.5 amps (with these assumed numbers)?

No, like I said, most amp ratings are qualified at a given impedance (8 ohms). If that amp was capable of 25 watts into 4 ohm loads, then great. Often it's current that's limiting the ability though. It's not the watts per se, but how we get there. Every amp has a maximum current and voltage limit.

Maybe an amplifier that can put out 25 watts rms means that 25 is just the average with no head room? And that the amps are cold and hard (need to be there or else) rated?
Am I at least heading in the right direction?

25 watts RMS (RMS is a pretty useless qualifier too) doesn't include headroom. Headroom is usually there.

What you often see is 2 extremes. Amps that use the qualifying figures to inflate the apparent power that the amp can deliver, or amps that purposely underrate their power.

[MGFalc]

This is a great topic, glad someone brought it up as its been driving me crazy trying to think about it and explain it to people over the years! I wish amplifiers were described in amps and not watts, it is an AMPlifier for goodness sake! but hey, thats just the number hungry public wanting something to talk about.

The problem with amplifiers ratings (and a lot of electrical ratings in general) is as I think kexodusc was trying to get at, is that there are too many variables thrown into the equation. Your speakers change in frequencies (unless you are playing a test tone and even then...) almost every millisecond and that calls into question different variables with regards to current, power, voltage required by the amplifier.

Generally most amplifiers are described with a power rating at 8 ohms, however there is no real law on this ie 10000W but at 0.0006 ohms can be said. It is getting an amplifier that can produce power without distortion and keeping stable (ie not cooking and clipping) is the key factor to think about. It is a fact that a lot of amplifiers have inadequate power supplies and hence the voltage can in turn upset these 'variables' that we've just been discussing also, so the 10000W at 0.0006 ohms aint gonna last for very long!

It was with the musical fidelity 'the preamp' that goes down in the books as the amp that changed our thinking of an amplifier being just a means to amplify...so maybe its not just mathematical equations after all!?

[GMichael]

[QUOTE=kexodusc]

No, like I said, most amp ratings are qualified at a given impedance (8 ohms). If that amp was capable of 25 watts into 4 ohm loads, then great. Often it's current that's limiting the ability though. It's not the watts per se, but how we get there. Every amp has a maximum current and voltage limit.
QUOTE]

Aha! I see what I missed now. Ratings are given at a set impedance. I missed that point.

Thank you Kex. The fog is lifting.

[MGFalc]

Quote 'Kexodusc'

"No, like I said, most amp ratings are qualified at a given impedance (8 ohms). If that amp was capable of 25 watts into 4 ohm loads, then great"

Yes and No. As I was saying, most amps can produce large amounts of power and current into certain ohmage, but for how long exactly...! a good amplifier is all about control.

[GMichael]

So let's look at the RX-V2500 (me) vs. the RX-V4600 (wanna-be). Both are generously rated at 130 watts rms @ 8 ohms. But the 4600 can take dips down to 2 ohms because it can put out more Amps. The "power" is rated the same but the Amps are way more for the 4600. If the 2500 were to hit a 2 ohm dip the amps needed would go too high for it and our arch enemy, clipping occurs.
This grasshopper thanks his master.........

[MGFalc]

note the internals of both amps:-

http://www.audioholics.com/productre...0inside_lg.jpg

and...

http://www.audioholics.com/productre...0inside_lg.jpg

the 4600 has clearly larger capacitors and a higher spec transformer than the 2500 (although still no toroid) and thus can take more of a 'battering'. I suppose thats what your money gets you! although for the money, id be tempted on the rotel processor/power amp offering, look at that beautiful toroid mmmmm

http://www.hometheaterhifi.com/volum...de-chassis.jpg

enjoy!

[GMichael]

note the internals of both amps:-

http://www.audioholics.com/productre...0inside_lg.jpg

and...

http://www.audioholics.com/productre...0inside_lg.jpg

the 4600 has clearly larger capacitors and a higher spec transformer than the 2500 (although still no toroid) and thus can take more of a 'battering'. I suppose thats what your money gets you! although for the money, id be tempted on the rotel processor/power amp offering, look at that beautiful toroid mmmmm

http://www.hometheaterhifi.com/volum...de-chassis.jpg

enjoy!

Very nice. Ya gotta love power. Oops, I mean Current.

[kexodusc]

Quote 'Kexodusc'

"No, like I said, most amp ratings are qualified at a given impedance (8 ohms). If that amp was capable of 25 watts into 4 ohm loads, then great"

Yes and No. As I was saying, most amps can produce large amounts of power and current into certain ohmage, but for how long exactly...! a good amplifier is all about control.

Good point. Which is why I like FTC ratings. Max power all channels driven for 5 minutes, or 1/8 power for 1 hour...how's that for control?

[Florian]

When you buy a power amp, try to find all the info of the unit and not just something like 700 wpc or a number like that.

This is from a good stable amp and nice description.

Bandwidth	0 - 300,000 Hz (+0/-1 dB, with input filter)
Phase response error	<10 (0 - 20,000 Hz)
Gain (balanced)	29.5 dB max. (x 29.9)
Gain (unbalanced)	29.5 dB max. (x 29.9
Minimum Power Output	>2x 160W into 8 Ohm, THD<0.01% >2x 280W into 4 Ohm, THD<0.01% >2x 420W into 2 Ohm, THD<0.01%
Output voltage/current, max	36V / 43A
THD+N (IHF-A)	<0.01% (1W into 8 Ohm, 1 - 20,000 Hz)
IMD	<0.010% (70W into 8 Ohm)
S/N ratio (IHF-A)	>120 dB (1W into 8 Ohm)
Slew rate	>100V/µs balanced >50V/µs unbalanced
Damping factor	>2000 (1 - 1,000 Hz)

[Geoffcin]

When you buy a power amp, try to find all the info of the unit and not just something like 700 wpc or a number like that.

This is from a good stable amp and nice description.

Looks like Krell specs.

The real fact of the matter is that if you never push an amp into clipping while listening, then even an amp that DOESN'T have monster specs like this one will still perform well. Most people just look at the specs, but that's really not what the whole story is. When hooked up to a speaker, an amp becomes a closed system. High sensitivity speakers may not benefit from a high current amp, while one might be absolutely nessassary for a speaker that presents a difficult load.

[Florian]

Its not a Krell or any other fat name amp. Just a good one

-Flo

PS: Cant be a Krell, it doesnt double down. The Krells double down on their specs.